1. Field of the Invention
The invention relates to a lumbar spine disc replacement apparatus with accompanying stabilization for the spine and method. The present invention is an artificial disc that can be implanted by a posterior surgical approach that can be used with devices and methods for dynamic stabilization of spinal vertebrae in a manner that permits motion within physiological normal range. The device provides mechanical resistance and support when the spine attempts to move beyond the desired limits.
2. Description of the Prior Art
Low back pain is an extremely important and costly public health issue, accounting for a significant proportion of the health care costs of modern industrialized nations. Conservative or non-operative treatment programs are the mainstay of therapy for this condition. This approach includes physical therapy, exercise programs, cognitive behavioral therapy, anti-inflammatory medications either orally or by spinal injection, and by modification in the work place environment and activities of daily living. Sometimes, however, pain and disability persist and surgical intervention becomes an option.
In general, there are two distinct but overlapping clinical problems, which often occur together. The first is termed sciatica, which is caused by compression of a spinal nerve either as the result of disc protrusion or bony compression. The second is axial joint pain, which results from painful degeneration of the joints of the spinal motion segment. A spinal motion segment consists of two vertebrae which are jointed together naturally by three joints: the intervertebral disc in the front of the spine; and the paired symmetrical facet joints in the back of the spine. The surrounding ligaments, tendons and muscle tissues are also important components of the spinal motion segment.
The surgical treatment for sciatica involves the physical unpinching of the compressed spinal nerve by removing the bulging disc or the bony spurs either alone or in combination. By taking the pressure off of the nerve, the irritation is eliminated and the sensation of pain and/or tingling numbness in the leg is relieved. Hopefully too the loss of strength in the affected muscles will also disappear in time. Unpinching a nerve, however, is not expected to have much affect on any low back pain which may be coexisting.
The surgical option for axial joint pain has traditionally involved a lumbar arthrodesis, also known as a fusion. A fusion operation is designed to stop and eliminate all motion in the spinal motion segment by destruction of some or all of the joints of the spinal motion segment. Bone graft material as well as rigid implanted fixating devices are employed for this purpose. By eliminating movement at the symptomatic spinal motion segment the expectation is that the low back pain will be reduced. The disadvantage to this approach is the loss of spinal flexibility. Additionally some authorities believe that a rigid fusion will place increased stress on neighboring spinal motion segments, thereby accelerating the degenerative and aging process. If these adjacent motion segments become symptomatic, then additional surgical intervention could become necessary on these previously non symptomatic areas.
One alternative approach to a rigid spinal fusion is the concept of joint replacement, either partial or total. The strategy is similar to joint replacement surgery in other areas of the body, such as with advanced degenerative changes in the hips or knees. In the spinal area, this involves the removal of some or all of the joints of the spinal motion segment accompanied by replacement with a mechanical device designed to replicate the function of the joint that had been removed.
Many examples of devices designed to replace the intervertebral disc or the facet joints exist. The most common approach to designing artificial spinal discs has contemplated surgical implantation using an anterior approach. This procedure requires an access surgeon skilled in mobilizing the large and dangerous blood vessels that obscure the site for insertion into the front of the disc. By nature of the anatomy, it is difficult or impossible to see and correct any pinched nerves when operating in this approach. Furthermore, extremely accurate placement of the artificial disc is essential to allow for the proper functioning of the implant. Damage to internal organs in the abdominal cavity, and damage to small nerves on the front of the spinal column resulting in retrograde ejaculation are additional concerns. Perhaps the greatest drawback to the anterior approach is the question of dealing with implant failure or displacement.
Revision surgery in this setting is recognized as extremely dangerous and even life threatening due to the scar tissue that has developed around the blood vessels located in front of the spine. Mobilizing these blood vessels the first time when the artificial disc is being initially inserted is challenging, especially because of the wide exposure needed to be certain that the disc is correctly positioned. Mobilizing these same blood vessels for repeat surgery carries a very high risk of serious complication or even death. Interference with the circulation to the lower limbs can have devastating consequences. Massive blood loss from tears in the veins stuck down by scar tissue can be fatal. Even if only a small number of anteriorly placed artificial discs require removal, the percentage of complications is anticipated to be unacceptably high.
For this reason, a posterior approach for implantation of an artificial disc for the lumbar spine would be preferred by many spine surgeons. By its nature, this approach permits and even mandates complete visualization of the nerves, so that any compressed neural structures can be unpinched. Furthermore, the potential abdominal complications, including damage to internal organs, possible retrograde ejaculation, and the risk to the major blood vessels supplying circulation to the lower limbs are avoided.
The main risk of the posterior approach is to the neural elements themselves. These include the nerve roots that are exiting the spinal canal as well as the central grouping of nerve roots called the cauda equina. The risk to these structures occurs during placement and surgical implantation of the device. Should the implant become dislodged, move, or migrate, then those structures are again at risk.
The first risk, occurring at the time of surgical implantation, can be minimized in two ways. The first is by the design of the implant, and the second is proper surgical technique. This requires adequate exposure of the posterior side regions of the disc, so that the implant can be inserted without any damage to the neural elements.
In order to achieve this goal, it is necessary to remove enough of the bony structures covering the lateral regions of the disc. This results in the surgical destruction of the facet joints, so that generally facet joint replacement devices are utilized in conjunction with artificial discs implanted using a posterior approach. The second major risk of this type of implant is the risk of subsequent dislodgment or migration, which could also damage the neural elements. This hazard is avoided by insuring that the posteriorly placed artificial disc is firmly attached and anchored to the vertebral bone.
There are a multitude of Patents and Patent Application Publications pertaining to artificial disc prostheses designed for implantation via an anterior or transabdominal approach. However implants intended for posterior insertion in the disc space are not as common. Devices intended not to replace the entire disc, but only the internal cushion portion called the nucleus pulposus are less technically challenging. Examples of inventions intended to accomplish this limited objective are disclosed in U.S. Pat. Pub. Nos. 2006/0064172 and 2006/0100304. Shape memory material is inserted through a relatively small opening in the posterior aspect of the outer thick layer of the disc called the annulus fibrosus. The material then expands or uncoils filling the cavity in the center of the disc where the nucleus pulposus formerly resided. The material is intended to recreate the cushion or shock absorber characteristics of the nucleus. The strong outer ligamentous layer of the disc, the annulus fibrosus, continues to provide strength and stability. Furthermore, the facet joints are left undisturbed.
There are several other devices that are designed for partial rather than total disc replacement. By partial disc replacement, only the inner component called the nucleus pulposus is replaced. However, there are several drawbacks to the strategy of partial disc replacement. There is a high risk for displacement or extrusion, since there is no firm anchoring strategy for the implants. For one thing, the degenerative processes that result in the need for surgical intervention rarely affect the nucleus pulposus in isolation. Indeed the other components of the spinal motion segment joint complex are often similarly affected. Therefore treatment aimed at only part of the problem is likely to be incomplete. There are also issues relating to the geometric special orientation of the nucleus replacement material, as well as the potential for later extrusion or displacement of the inserted material.
An approach to total disc replacement is disclosed in U.S. Pat. No. 6,419,706 which describes a cylindrical cage designed to screw into the endplates above and below the disc space. It is composed of a metal shell that is divided in two mobile upper and lower halves for anchoring into the endplates. This metal jacket surrounds a flexible core. The core is a “viscoelastic” material—suggestions include a silicone polymer. This design could be inserted using a posterior approach in the same manner that cages filled with bone graft material are placed when a spinal fusion is the desired outcome. The problem of extrusion of the central core is an issue, as is the shortcomings associated with a cylindrical implant. Typically these devices require more retraction and therefore potential injury to the neural elements, particularly when a large size is required due to a large disc space with preserved height. This is in contrast to more rectangular shaped implants, where height can be increased without increasing width. The other unavoidable aspect of the screw in cylinder design is the destruction of the endplates necessitated by the action of the screw threads. This could result in subsidence of the implant with subsequent collapse and narrowing of the disc space over time with attendant loss of motion.
U.S. Pat. Pub. Nos. 2006/0085073 and 2006/0085074 reveal a device intended to replace not only the entire disc but also both paired facet joint. It contemplates a biocompatible thermoplastic polyurethane or high performance nylon “balloons” inflated with a fluid in a closed hydrodynamic circuit. There is a fluid connection not only between the rod-like balloons attached to the pedicle screws, but also with the inflatable or expandable element filling the disc space. Loss of pressurization over time is an obvious concern with respect to that design.
In U.S. Pat. Pub. No. 2006/0085076, there are paired implants placed from a posterior direction on either side of the disc space. Each disc implant is composed of two components, which mate with one another by employing a shallow asymmetrical ball and socket design, which permits some translation as well as flexion and extension between the two components. Each component is secured to the underlying vertebral endplate with a single angled screw with a recessed head. It is stated that the surface of the prosthesis can be provided slightly roughened so as to increase bonding of the same with bone and/or one or more surface coatings can be provided thereon, such as for example, hydroxyapitite or plasma spray. The posterior elements replacement device is a telescoping arch which is anchored to pedicle screws. The sliding members articulate somewhat loosely which allows for “small degrees of rotation and side to side flexion.” This design gives the illusion of bending as the rod lengthens. Problems with this design include the potential for posterior extrusion of the implant. Each half of the paired device is secured only with a single bone screw. Additionally there is little to prevent dislocation of the shallow ball and socket articulations.
U.S. Pat. Pub. No. 2005/0283247 discloses a design for posterior artificial discs with the ability to expand the implant after it is inserted into the disc space. A single banana or boomerang shaped implant is used rather than two symmetrically paired devices. The expansion is achieved by a number of different options, including a cam shaft design which permits sequential distraction by turning a screw mechanism built into the side of the device. In some of these embodiments it appears that the expansion may not be readily reversible. The disc replacement prosthesis is intended to be used in conjunction with a posterior dynamic stabilization system replicating the function of the facet joints. The elongated member connecting pedicle screws is constructed to allow for movement within the pedicle screw heads as well as a ball and socket joint at the midpoint of the rod, which allows for bending at that point. The mobility at the pedicle screw heads is permitted by wiggly attachments, either by a mismatch between the size of the pedicle screw heads and the rod, or by a mismatch between the screw heads and a receiving hole in the flat plate-like terminus of the rod. It is not disclosed that the rod-like member has the capacity to elastically increase in overall length. An increase in the distance between pedicle screw heads is essential if spinal flexion is to occur in a relatively normal fashion with an anteriorly located instantaneous axis of rotation. An additional potential issue is the difficulty of placing a single implant of this shape in the exact center of the disc. Perhaps of even greater concern is the potential for dislocation between the upper and lower element at the ball and socket articulation. It is stated that in some embodiments an “elongated member” may couple the upper and lower elements.
Another expandable design is disclosed in U.S. Pat. Pub. No. 2005/0261769. Two metal shells are jacked open and apart using a gear mechanism. Several options for the core element situated between the metal shells are discussed. A drawback with all of these options is the difficulty that would be encountered in removing the device, should revision and extraction ever be required.
Since posterior placement is preferable to anterior placement, a need exists for a posterior disc replacement implant device that can be securely positioned without risk of displacement or migration. The device should replicate the primary functions of the disc allowing for flexion, extension, and modest rotation. Furthermore the device should be removable in a safe manner, facilitating replacement or revision surgery if it should be required.
It is anticipated that the patient's facet joints will be either partially or totally removed in preparation for implantation of the invention into the disc space. This is required in order to avoid excessive traction or damage to the nerve roots during insertion of the invention, and to permit placement of the attachment shaft in a location distant from the nerve roots. For this reason the device according to the invention is best used with dynamic spinal stabilization system or devices.
Dynamic spinal stabilization system or devices augment the existing joints of the spinal motion segment by providing additional strength and support by some form of mechanical resistance. The objective is to permit some motion within a physiologic range, yet relieve the symptomatic painful joints of a portion of the physical stresses. The symptoms are improved as the device shares some of the load placed on the spine and protects against excessive or abnormal motion. Additionally some authorities believe that dynamic stabilization reduces the probability of accelerated degenerative changes on adjacent motion segments. Hopefully pain is lessened, flexibility is preserved and future problems at neighboring motion segments are reduced. The dynamic stabilization system can also be used in combination with a joint replacement device.
Spinal deformity is another potential application for dynamic stabilization. Scoliosis, or abnormal curvature of the spine, causes a rotational and side bending of the spine resulting in an abnormal shape and contour of the involved areas. Traditionally a spinal fusion operation is performed in an attempt to correct the curvature. An alternative approach is dynamic stabilization.
Many types of spinal stabilization devices are known. In U.S. Pat. No. 4,448,191 a flat metal band of titanium alloy is attached to the side of the spinous processes to exert a chronic dynamic corrective force to reduce the rotational as well as the lateral deformity of scoliosis. A method utilizing flexible rods of stainless steel is described in U.S. Pat. No. 4,697,582. Here the rods are also attached to the base of the spinous processes, and the guidance attachments allow axial movement of the rod permitting longitudinal spinal growth.
A similar concept for allowing the sliding of long rods either caudally or cranially is shown in U.S. Pat. Pub. No. 2004/0143264 which utilizes sleeves. A design employing springs is provided in U.S. Pat. No. 5,672,175. In this design two rods are attached to the spine with pedicle screws, with compression resisting springs on one side and extension resisting springs on the other side. The rods are fixed at the midpoint of the deformity but can slide through the connectors at either end—enabling longitudinal growth as well as rotation about any horizontal axis. This design also contemplates electronic micromotors to enable adjustment in the tension by moving the position of a stop. A device placed anterior to the spine is described in U.S. Pat. No. 6,296,643. Plates are positioned along the front of the vertebral bodies using bone screws. These plates are then connected using a cable, synthetic ligament, or flexible rod.
Axial joint pain resulting from degenerative changes in the spinal motion segment is difficult to treat. Sometimes dynamic stabilization is employed as a stand alone strategy. The concept is that by sharing some of the load, the device relieves the spinal joints of stress and thereby reduces symptoms. Some designs rely on the presence of intact and preserved anatomic features of the vertebra, such as the midline spinous process arising from and projecting back from the lamina of the vertebra. One of the older and simpler concepts is disclosed in U.S. Pat Pub. No. 3,648,691. A flat strip is clamped to several spinous processes. A flexible non-toxic material such as vinylidene fluoride was stated to be preferable to cast or machined metal straps. In U.S. Pat. No. 5,011,484 a plastic insert is described which fits over and between the spinous processes to restrict but not entirely prevent movement. The suggested material is polytetrafluoroethylene with a low friction coefficient to facilitate the sliding of the spines of the vertebrae inside the inserts. A semi-flexible intraspinous block is described in U.S. Pat. No. 5,609,632. This design also contemplates a flexible ligament composed of Dacron® (polyethylene terephthalate) wrapped around the spinous processes. In U.S. Pat. No. 6,440,169 a titanium alloy leaf spring is placed between the spinous processes, with one variation including a solid core of a viscoelastic material such as polyurethane or silicone.
Another device mounted on the spinous process is detailed in U.S. Pat. Pub. 2002/0095154. This is a design consisting of a compression spring, with another embodiment utilizing a piston/cylinder design and a flexible housing with a gas or liquid working the piston instead of the spring. All of these devices depend on intact spinous processes, and cannot be used when these structures are small or have been surgically removed.
In U.S. Pat. Pub. No. 2006/0084991 a design is presented which is also envisioned to be positioned in part between existing spinous processes, although the inventors state it can also be implanted after laminectomy and removal of the spinous processes. Transverse rods are contoured and bent toward one another to permit location between the spinous processes and also to allow for placement of an articulating joint and/or a central spacer between the two transverse rods. In addition, elastic elements are used to further join together the transverse rods. It is suggested that the elastic elements could be preferably formed from a biocompatible polymer, such as polyurethane, composite reinforced polyurethane, silicone, or other materials.
Another approach to strengthening the spine yet preserving motion is the use of artificial ligaments. The use of a flexible ligament made from Dacron® attached between pedicle screws is presented in U.S. Pat. No. 5,092,866. A more complicated design also intended for use between pedicle screws is described in U.S. Pat. No. 5,180,393. Braided multifilament yards of retractable polyester are arranged in two separate layers: a longitudinal primary layer in a figure-of-8 pattern covered by a transverse secondary winding. The first layer resists extension and the second layer resists compression. Other concepts include the notion of augmentation of the anterior longitudinal ligament in the front of the spinal column. A synthetic anterior longitudinal ligament composed of ultra-high molecular weight polyethylene in the form or a single strand, cable, tube, or patch is presented in U.S. Pat. Pub. No. 2002/0107524. A mesh design is proposed in U.S. Pat. Pub. No. 2002/0120269, with a variety of metal or fiber materials suggested as options. A wide range of possibilities mostly involving anterior flexible bands was revealed in U.S. Pat. Pub No. 2002/0120270. Cross coupled bands between pedicle screws in an “X” pattern to help prevent rotational forces on facet joints, used in association with flexible “dampers” attached to pedicle screws, were disclosed in U.S. Pat. Pub. No. 2002/0133155. Although some flexibility may be permitted by these ligament inventions, elongation is generally prohibited by these designs due to the inherent lack of elasticity.
Solid flexible members positioned between pedicle screws is another design consideration. A flat or oval flexible strip composed of carbon fiber reinforced plastic is described in U.S. Pat. Nos. 4,743,260 and 5,282,863. A variety of polymers are suggested, and manufacture using a replamineform process is recommended. This results in porosity of the strip, and it is anticipated that fibroblasts will grow into these porosities, augmenting its fixation but preserving the flexibility. A solid flexible rod composed of aromatic polycarbonate-polyurethane based material such as Bionate® or Chrono-Flex® is suggested in U.S. Pat. Pub. No. 2003/0220642. A titanium rod divided by a flexible joint made of “organic silicone compounds” is described in U.S. Pat. Pub. No. 2004/0049189. In U.S. Pat. Pub. No. 2003/0171749, the central portion of the rod is divided in half lengthwise. Bending of the rod is permitted only along the sagittal plane dividing the bifurcated area, thus limiting bending in other planes. Several flexible rod-like members constructed from hollow tubes with slits cut in a spiral pattern are presented in U.S. Pat. Nos. 6,986,771 and 6,989.011. Two or more tubes with different spiral patterns are fit snugly one within the other. Tension bands attached to the side of the tubes can provide resistance to motion. Other embodiments include a solid central flexible rod, braided wire, flexible plastics and rubber based materials. All of these flexible straight strip, rod, and rod-like designs permit bending, but once again elongation of the flexible member is not possible. Another kind of solid flexible rod is disclosed in U.S. Pat. Pub. No. 2005/0277922, in which a molded rod composed of a compressible substance was surrounded by a molded material resistant to elongation. A large number of materials were suggested, including a rubbery polymer for the compression element and silicone-polyurethane for the tension element. It is unclear how much elongation and bending would result from this combination.
A straight and rather large diameter device for use as an elastic “damper” is shown in U.S. Pat. No. 5,540,688. This invention is constructed of a core material loaded in elongation, and a sleeve or jacket loaded in compression. Three embodiments are presented, and the materials suggested included a “bio-compatible elastomer.” In contrast to other straight rod-like designs, this invention would permit elongation of the member. However, bending is prohibited, and flexion and extension of the spine apparently depends upon the function of a ball and socket articulation with the pedicle screws permitting free polyaxial movement.
A flexible “U” shaped rod is described in U.S. Pat. No. 5,415,661. Detailed manufacturing options are discussed and the preferred embodiment is identified as a carbon reinforced plastic. Specifically the carbon fibers are oriented in the optimal alignment and density using prepreg tapes, and net compression molding using polyetheretherketone as the polymer is employed. The design in the shape of a “U” would permit elongation between the pedicle screw heads, which is essential in permitting forward flexion of the spine with a relatively normal instantaneous axis of rotation. Unfortunately the design is impractical because of the physical space limitations in the operative field. The distance between the pedicle screw heads is quite small, especially between L5 and S1. Furthermore the anteriorly projecting “U” shape would impinge upon the posterior bony structures of the spine or the nerve roots. A similar concept involving a more complex “inverted T” shaped bend in a flexible rod is presented in U.S. Pat. No. 6,966,910, but suffers from the same limitations.
Various spring designs have been incorporated into inventions designed to preserve motion yet add support to the spine. An invention designed for use in the front of the cervical spine utilizes a series of leaf springs which resembles an accordion, and is revealed in U.S. Pat. No. 6,293,949. A shape memory alloy is suggested as material, and specifically nitinol is recommended. This is an alloy of titanium and nickel with a low corrosion rate, excellent wear resistance, and minimal elevations of nickel levels in the tissues in contact with the metal. The presence of vital vascular structures makes this device unsuitable for the anterior lumbar spine.
Several designs utilize springs with rod-like members in order to permit elongation and compression of the rod, often incorporating a piston and cylinder component. A relatively simple device is disclosed in U.S. Pat. Pub. No. 2004/0049190 in which a rod is surrounded by a spring. The rod is rigidly fixed to one pedicle screw, but sliding is permitted through the attachment to the other pedicle screw. The rod is coated with ultra-high molecular weight polyethylene to permit sliding. Either a compression or an extension spring can be used. Bending of the construct, however, is not allowed. A modification to permit both elongation and bending is disclosed in U.S. Pat. Pub. No. 2006/0036240. A spring is associated with a telescoping rod with a curved track. With elongation, one component of the rod slides along the curved track permitting an imitation of bending. A telescoping rod on a curved track without an associated spring is presented in U.S. Pat. Pub. No. 2006/0085076. The device is meant for use in conjunction with posterior artificial disc implants. This again permits elongation or shortening with the illusion of bending.
Another approach to the twin problems of achieving not only length variability but also flexibility is provided in U.S. Pat. Pub. No. 2005/0283247. This is designed for use with a posteriorly positioned artificial disc replacement device. The elongated member connecting pedicle screws is constructed to allow for movement within the pedicle screw heads as well as a ball and socket joint at the midpoint of the rod, which allows for bending at that point. The mobility at the pedicle screw heads is permitted by wiggly attachments, either by a mismatch between the size of the pedicle screw heads and the rod, or by a mismatch between the screw heads a receiving hole in the flat plate-like terminus of the rod.
In U.S. Pat. No. 7,029,475, the device employs compression springs in a telescoping piston system. This invention is adjustable so that maximum resistance to motion in elongation and shortening is achieved in the neutral zone. Outside of the neutral zone the resistance provided by the invention is less. Bending of the device is not possible, so in order to allow spinal flexion, special mobile articulations with the pedicle screws are required to permit active angular movement at the points of attachment.
Hydraulic circuits are yet another design concept. In U.S. Pat. Pub. No. 2003/0055427, a compression spring and a piston containing a hydraulic circuit with the reservoirs are placed in the disc space. Bending of the device is not permitted, so flexion and extension of the spine once again apparently depends upon the function of a ball and socket articulation with the pedicle screws permitting free polyaxial movement. Yet another design envisioning the use of a complex hydraulic circuit is disclosed in U.S. Pat. Pub. Nos. 2006/0085073 and 2006/0085074. Inflatable elongated members composed of a type of biocompatible thermoplastic polyurethane are attached to the spine via pedicle screws. These fluid filled rod-like elements are connected through a closed hydraulic circuit to another inflatable element having the shape of and taking the place of the intervertebral disc.
An attempt to attain flexibility along with the potential for lengthening and shortening is described in U.S. Pat. Pub. No. 2003/0220643. Such a device would not depend upon complex moveable articulations with the pedicle screws. Several options are discussed. One embodiment contains two concentric springs separated by a rigid or semi-flexible tube. Another embodiment is an elastic cord surrounded by the extension block tube. Yet another variation discloses a telescoping flexible tube containing a spring, with the tube composed of a shape memory alloy such as Nitinol, or polyethylene. These designs share the common goal of allowing bending and elongation with a stretchable element, but preventing buckling of the stretchable element with a surrounding tube.